R18-1998.pdf (396.0Kb)

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International Council for the Exploration of the Sea

ICES C.M. 19981R:18 Theme session on mesoscale physical phenomena and biological production

INFLUX OF ATLANTIC WATER AND FEEDING MIGRATION OF HORSE MACKEREL

Abstract

By

Svein A. Iversen, Morten D. Skogen and Einar Svendsen sveini@imr.no; morten@imr.no, einar@imr.no

Institute of Marine Research P.O.Box 1870 Nordnes

5024 Bergen, Norway

The Norwegian fishery for horse mackerel in the Norwegian economical zone is unregulated and thereby reflecting the availability of horse mackerel in these areas. The- Norwegian fishery is exploiting western horse mackereL The availability of horse mackerel for this fleet is depending on extensive lr..igration of we stem horse 'lrwckerel. The ~~orvVegian catches have increased significantly since 1987 and the catches for the period 1982-1997 are cOlnpared with the modeiied in!lux of Atlantic water to the northern pan of the North Sea. This modeiied influx correlates strongly with the fluctuations in the Norwegian horse mackerel fishery during this period:

The fIShery and migration of horse mackerel

Norway has in later years been the main fishing nation for horse mackerel in the North Sea and Norwegian Sea. This fishery is mainly carried out in the Norwegian economical zone of the northern part of the North Sea but also to a smaller degree in the southern part of the Norwegian Sea in the second half of the year, particularly in October. Tne fishing area is relatively small usually between 58° - 63° north and between 2° east and the Norwegian coastline. This fishery is considered to exploit the western stock (Anon., 1990, 1991). The fishery is carried out by Norwegian purse seiners. This fleet adapts its effort in this fishery according to the actual availability of horse mackerel in the Norwegian economical zone. This means in years with low availability of horse mackerel for the purse seiners they will leave tbis fishery. The Norwegi::tn fishery is not regn 1 ated witlrin t..he Norwegi::tn zone and is therefore considered to reflect the availability and abundance of horse mackerel in this area during the autumn.

Based on results from egg surveys and the spatial and temporal distribution of the fishery ICES a..re considering the horse mackerel in the North East Atlantic area as separated into tl1ree management units (stocks); soutbem-, westem-, and North Sea horse mackerel (Anon.,

1990, Anon.,1996a). They are na..TJled after their spawning areas. The western horse mackerel are spawning west and south west of Ireland It is assumed that western horse mackerel at least since the early 19808 has adopted the same migration pattern as western and southern mackerel (Anon., 1990, 1991, 1996b, SEFOS, 1997). After spawning (late July) in the area southwest and west of Ireland the fish migrate northwards to the Norwegian Sea and North Sea. It seems that the horse mackerel enter this area later than mackerel and also leave the area earlier. The main Norwegian fishery for horse mackerel is carried out in October, while the mackerel is available in considerable quantities in the same area during the third-, fourth- and some years also in the first part of the first quarter of the following year before they migrate

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Norwegian iagging experiments since 1969 (Bakken and ,\Vestgard, 1986, Iversen a..Y'ld Skagen, 1989) and for southern mackerei by Spanish tagging experiments in 1994 (Uriarte, 1995). In the later years Spain has tagged horse mackerel, but so far no tags have been recovered.

The Norwegian fishery for horse mackerel in the Norwegian economical zone was quite small until 1987 when the catches increased to 15000 t and further to a maximum of 127 000 t in 1990 (Figure 1). j'.fter that the catches declined by 60 % in 1991 and then increased in 1992 and in 1993 to the same level as in 1990. Since then the catches dropped to about 95 000 t in

1994 and 1995, t015 000 t in 1996 and increased to 46 000 t in 1997. Little is known about the age composition of the Norwegian catches before 1991. However, based on length frequencies in the catches, and Dutch age readings, it seems that the extremely rich 1982 year class have dominated the Norwegian catches since 1987. Only fish older than five years seems to undertake the long journey to the waters fished by Norway. Surveys in the Norwegian Sea have demonstrated that the largest horse mackerel are migrating the longest distance (HoIst and Iversen, 1992). The 1982 year class has also dominated the catches of western horse mackerel in ot...l:ler areas since 1984 (Anon.,. 1997). ·The tOt3J catches of western ,horse mackerel are known for the period -1982-1996 (.l\non., 1998) and increased from about 40000 t in 1982 to SiQ 000 l in 1995 (Figure 2) after that the catches dropped to about

400000 t in 1996. The total international catches for western horse mackerel in 1997 seems to be at the same level as in 1996 (Iversen pers. corn.). Particular since 1993 it seems that horse mackerel was relatively more available for fishing fleets working in the western area than for the Norwegian fleet.

Factors trigging the migration

The migration is undertaken for feeding purposes. The changes in the fishery in the

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NorwegIan Ilsfllng area anu Luereuy cuanges in rrJ.1gratlon llligut:ue exp.Latnev ...,y at ... east hvo factors:

1. Variations in stock .size

2. Changes in hydrographical conditions affecting the migration and catchability directly or indirectly by affecting the feeding condition

The ICES Working Group has estimated the spawning stock biomass (SSB) for the period 1982-1997 by the ADAPT analysis (Anon., 1998) (Figure 3). The spawning stock has been measu..-oo. ever/third years-dur ... Tlg the western-mackerel and horse-mackerel surveys (Anon.~

i996a). Particularly during the period pr~vious 1992 the i~~~AJlT based SSB estimates a..re overestimated considerably compared with the eStillmtes based on the egg 'Su..--Yeys. The ADAPT analysis is carried out particularly to calculate the development of the SSB after the last eggsurvey in 1995. The spawning stock has declined since 1988 because no new strong year classes have been observed to recruit the stock since the very rich 1982 year class. It is impossible to tell if the size of SSB is the only or main factor trigging the migration to the North Sea and Norwegian Sea. Neither is there so far much knowledge of what minimum level the stock size has to be for trigging this. According to the egg survey based SSB

estimates (Figure 3) it seems that the SSB has to be larger than the level estimated in 1986, i.e.

750000 t before migration is trigged. The very numerous 1982 year class was probably previous 1987 individuaily io small io undertake that long 1lJigration.

Since the horse mackerel caught by the Norwegian fleet is migrating into their fishing areas from the western spawning area the availability of fish for this fleet might be linked to the inflow of Atlantic water. This has been estimated by the NORWegian ECOlogical Model

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The modei design

The NORWECOM (Skogen 1993, Skogenet al., 1995) is a coupled physical, chemical, biological model system applied to study primary production and dispersion of particles (fish larvae and pollution). The main weakness of 3-D modelling activities claiming to simulate nature is the lack of comparison with adequate real data. Therefore the extensive SKAGEX data set (Danielssen et al., 1991, Danielssen et al., 1997, Ostrowski, 1994) has been used to validate (Dee, 1994) the model (Svendsen et al., 1996, Skogen et al., 1997). The model has also been used to investigate long-term residual transports in the North Sea together with two other models (Smith et al., 1998), and taken 'part in several models to model intercornpa..rison studies (R~ed et ai., i989, Gusiafsson and J~nsson, 1995, Proctor_et al., 1997).

The circulation model is based on the wind and density driven primitive equation Princeton Ocean Model (Blumberg and Mellor, 1980, Melior, 1996). In the present study a 20 x 20 km horizontal grid covering the whole shelf area from Portugal to Norway, including the North Sea, h~. been used (Figure 4). In the vertical the model uses 12 sigma layers.

The forCing variables are six-hourly hindcast atmospheric pressure fields provided by the Norwegian Meteorological Institute (DNMI) (Eide et al., 1985, Reistad and Iden, 1995), 6-hourly wind stress (translated from the pressure fields by assuming neutral air-sea stability), four tidal constituents and freshwater runoff. To absorb inconsistencies between fOf1;ed boundary conditions and model results, a 7 gridcell ;;F!ow Relaxation Scheme" (FRS) zone is used around the open boundaries (Martinsen and Engdahl, 1987). In the lack of data on the surface heat fluxes, a "relaxation towards climatology" method was used (Cox and Bryan, 1984). During calm wind conditions, the surface temperature field will adjust to the climatological values after abollt 10 days (Oey and Chen, 1991). The net evaporation

The model·runs

The model is initiated from monthly climatologies (Levitus, 1982; Martinsen et aL, 1992) Jor

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mean (197 6^e 1997) modelled residual currents for winter (January,. February and March) at 10 m depth are given. Based on the 25 hourly mean current fields, average (daily) transports into the North Sea through an east-west section going from Norway to the Orkneys (along 59°17' N) has been computed. From these daily values, monthly transports are estimated for each of the modelled years (1976-1997). The winter volume flux is defined as the mean of the

January, February and March fluxes.

Conclusion

The estimated fiuxes and catches of mackerel since i 976 are shown in Figure 6. In the period of significant Norwegian catches (1987-1997) the catch level are strongly related to the influx level. Ittherefore seems that when the western horse mackerel stock is migrating, the amount of fish entering the Norwegian area are closely correlated to the influx of Atlantic water (Figure 7). This correlation might be used for predictive purposes. For 1997 the model

predicted an increase in t.l:!e catch from 15000 t in 1996 to about 70 000 t (Iversen et ~1., 1997)

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above the level trigging significant migration to the North Sea and Norwegian Sea, a modelled influx this winter of about 2.11 Sverdrup predicts a Norwegian catch of about 30 000 t during the coming season. Standard error of the predicted catches are calculated by SYST AT to l:Je 24 000 t,

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T ll.alt-!;Inl;!nn R nh:.cnn T !:Inn C;;;::upnrlcpn H' 1001 ~v dn:PV· ~nTT"la n,..""'l; ... ~n..,...,' ... , ... ..., ... , ~., ... , .... , ... ""' ... , ... " ... .l"I..l :t ... "' ... .1.0..1"-' p ... .LL.L.l.ll.l.U.U.}

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Hno-1n .... pr;THT 11 i=irfL':;;')7 ... 0 ... .0' ... .L, ... __ ... I •

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S ven sen, h., Hemtsen, d ---. - J., ... ~Kogen, ... lV1.1J., ^{Jr T"'I.} fiUli1UU:SVll\.., i ...11 __ ...l ... _.!lF n .1.>., ... au.u ....t 'l'\JI"n J.uartJ.u., ... ~ .... "' ... u,~ p . ... 1 QO'; ~/..., . ... Mnrlpl u . ..., ...

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Fig. 7 Norwegian horsemackerel catches versus winter volume fluxes 1976-1997.